Currently, there are many different designs for solenoids, although conventional solenoids generally all consist of a magnetizable rod plunger with one end of the plunger located in an electromagnet yoke which contains windings of magnet wire in the form of a coil. When power is applied to the windings, a magnetic field or flux is created. The linked portion of this flux (i.e. linking through the plunger) causes the plunger to be attracted towards the back of the yoke through the plunger air gap. While the power is applied, the plunger will be held in the closed position by the linked flux. When the power is stopped, the magnetic attraction is released. In most constructions, a mechanical device such as a spring will then return the plunger to its original open position.
Solenoids are in widespread industrial and consumer goods use, particularly automotive and electronics, as electro-mechanical switching devices. Solenoids can be mass produced in a variety of shapes and sizes. Usually, they are reliable when new. However, with extended use, reliability is a concern. An increase in coil resistance or friction between the plunger and the yoke may easily be enough to prevent the plunger from activating to the closed position. Dirt or wear can cause the solenoid to stick. Continuous opening and closing can cause the coils and thus the plunger to heat up. The resultant thermal expansion may then lead to jamming of the solenoid. If the plunger becomes stuck in the open position, continued power can burn out the coil windings. If the solenoid fails to release to the open position, damage may be done to a connecting component which it is controlling.
In the conventional solenoid construction, a large portion of the flux generated is unlinked, and is therefore not used in the process. This results in an inefficient use of the solenoid yoke during plunger movement. Another limitation of the conventional solenoid construction is the distance of travel of the plunger. For a given solenoid size, the maximum distance of plunger travel will be a function of the amount of unlinked flux. The greater the amount of the unlinked flux, the smaller the distance of plunger travel.
Because of the above limitations, and for reasons of unreliability and safety, many designers try to minimize the use of solenoids wherever possible.